The Universal Serial Bus ("USB") development as a standard for use with personal computers has been motivated by the necessity to provide for connection of the personal computer ("PC") to the telephone, the necessity for ease-of-use or the so called "plug and play" approach, and the need to facilitate the addition of external peripherals. For years, computer users have been limited in their ability to add devices to their computers.
The Universal Serial Bus of "USB" is a standard peripheral interface for attaching PCs to a wide variety of peripheral devices including keyboards, mice, scanners, printers, game controllers, modems and other peripherals. With USB, up to 128 devices may be interconnected.
All types of devices are now available as USB devices. No longer does a user have to worry about configuring his/her system. When a USB device is plugged in, the device is automatically detected, and the software is automatically configured. The USB is useable with video capture devices, keyboards, mice and scanners. USB devices require a central processing unit as a host unit.
The standards for USB are publicly available and are the result of a collaborative effort by major computer manufacturers. The Universal Serial Bus Specification revision 1.1 (Sep. 23, 1998) by Compaq, Intel, Microsoft and NEC sets forth connectivity standards for PC architecture. It sets forth a "fast, bi-directional, isochronous, low-cost, dynamically attachable serial interface structure that is consistent with the requirements of the PC platform of today and tomorrow." The specification defines an industry-standard USB and describes the bus attributes, the protocol definition, types of transactions, bus management, and the programming interface required to design and build systems and peripherals that are compliant with the standard. All USB devices connect to the USB through ports on specialized USB devices referred to as "hubs". Hubs are wiring concentrators. Attachment points to hubs are referred to as ports. Each hub converts a single attachment point into multiple ports. The architecture supports concatenation of multiple hubs.
The USB topology is such that the USB connects USB devices with a USB host in a tiered star topology. A hub is at the center of each star. Each wire segment of the USB is a point-to-point connection between the host and a hub or function, or a hub connected to another hub or function. The upstream port of a hub connects towards the host. Each downstream port of a hub allows connection to another hub or function.
In the USB topology there is only one host. A root hub is integrated within the host system to provide one or more attachment points. Additional attachment points to the USB are provided by hubs. Each hub may be connected to additional hubs and to functions which provide capabilities to the system, such as an ISDN connection, a digital joystick or speakers. All USB devices attach to the USB through ports on hubs.
The USB transfers signal and power over a four wire cable. All USB devices have an upstream connection, and USB cables have an upstream and a downstream connector. The upstream and downstream connectors are not mechanically interchangeable to eliminate illegal loopback connections at the hubs.
In accordance with USB standards, all attached devices connect to the host device through a single connector type using a tiered star topology. Personal computers typically have one or two USB ports. To provide for connecting to a number of devices, a cable connection is provided to a USB hub via a USB cable. Currently, hubs having four or seven ports are dominant, which give four or seven additional USB connectors. In addition, a USB cable may connect from the USB hub to one or more additional USB hubs. One of the limitations that applies to USB architecture is a distance limitation which is a 5 meter cable length, or its electrical equivalent, between devices.
A PC configuration using USB hubs and functions will require a number of different items to be cabled together. The cables interconnecting the hubs and functions creates an undesirable cabling appearance.